1. Field of the Invention
This invention relates to a contacting method in contact printing, such as, for example, a contacting method in which a photomask (mask) and a wafer are set in a predetermined relation in a step of bringing the mask and the wafer into intimate contact with each other and in which a pattern on the mask is printed on the wafer in the manufacture of semiconductor.
2. Description of the Prior Art
In a method of this type, the mask and wafer are held at a suitable interval, whereafter a vacuum is created in closed space formed between the mask and the wafer and atmospheric pressure is caused to act on the back of the mask, whereby the mask is bent toward the wafer side to thereby bring the mask and the wafer into intimate contact with each other. In this case, the initially set gap between the mask and the wafer has greatly affected the intimacy of the contact between the mask and the wafer.
That is, when the gap amount is set to a small value, the amount of flexure (the amount of curvature) of the mask is small. Therefore, good intimate contact is provided in the marginal portion of the mask and the so-called pitch error is small. The pitch error is a phenomenon that the patterned surface of the mask becomes lengthened by curvature of the mask and the mask pattern is printed on the wafer at a location thereon that deviates from the position whereat the mask pattern should be printed.
However, there has also been a problem that when relatively wide areas of the mask and wafer begin intimate contact with each other, the gas present near the central portion (for example, N.sub.2 gas or the like for protecting negative resist) is cut off from escape and shut in thereby causing deterioration of intimacy of the contact in the central portion. It is possible to wait for the escape of the gas, but this method would offer a problem in respect of the through-put of production. The poor intimacy of the contact in a minute gap generally leads to reduced resolving power due to diffraction of light.
On the other hand, when the gap amount is set to a great value, the mask bends greatly and begins to make intimate contact with the wafer from a small area near the central portion thereof. Due to the increased degree of vacuum, this area tends to gradually widen toward the marginal portion, whereby the above-described phenomenon in which the gas is shut in near the central portions of the mask and wafer during the intimate contact thereof is alleviated and the intimacy of the contact in the central portions is improved.
In this case, however, the mask is greatly bent over toward the wafer side and curved during the intimate contact thereof. Leads to a problem that due to the rigidity of the mask, the mask and wafer do not make intimate contact with each other in the marginal portions thereof. Therefore, the degree of intimate contact in the marginal portions is reduced to aggravate the quality of image and accordingly the chip collection rate in the marginal portions is reduced. Also, the fact that the mask is greatly curved toward the wafer during the intimate contact therebetween leads to a problem that the afore-mentioned phenomenon of pitch error becomes pronounced.